The response of turbidity currents to a canyon-fan transition: internal hydraulic jumps and depositional signatures

Turbidity currents often carve canyons into the continental slope, and then deposit submarine fans on lower slopes farther downstream. It has been hypothesized here that this slope decline can cause a turbidity current to (a) undergo an internal hydraulic jump near the canyon-fan transition, and (b) leave a depositional signal of this transition. These hypotheses are studied with a numerical model. Rapidly depositing turbidity currents need not undergo a hydraulic jump at a slope break. When a jump does occur, it can leave a depositional signal in terms of an upstream-facing step. A previous attempt to capture this signal failed because the current was treated as purely depositional. In the present model both sediment deposition and entrainment are included. An upstream-facing step appears when deposition dominates erosion. The step requires entrainment since the deposition rate is continuous through the jump, whereas the sediment entrainment rate is not. Therefore, the step is caused by enhanced net deposition due to reduced entrainment rate across the jump. Under certain circumstances, a single step can be replaced by a train of upstream-migrating cyclic steps, each separated by a hydraulic jump. The numerical model is verified against experiments, and then applied at field scale.